1. Field of the Invention
The present invention relates to a system using a control device for adjusting oxygen concentration in a gas of the system to a desired value, particularly to an atmosphere control system using a control device for controlling the oxygen concentration in the gas of the system.
2. Related Art Statement
Heretofore, in case when an easily oxidizable metal in a determined concentration of oxygen, particularly in a low concentration of oxygen, is subjected to heat treatment, such as, carbonization, firing, or baking of paste, or annealing, commercial standard gases, such as a high purity Ar gas, an N.sub.2 gas containing 2% of O.sub.2, etc., are purchased and used as atmosphere gases in furnaces for such heat treatments and annealing.
However, in cases when using the above commercial standard gases, a standard gas of a completely desired oxygen concentration is difficult to obtain commercially, and such standard gas of the completely desired oxygen concentration can be obtained only with a very high increase of cost.
In cases when a standard gas of an extremely low oxygen concentration has to be used, even an N.sub.2 gas, for example, which is considered not to contain oxygen, contains moisture or oxygen of a level of about 0.1-10 ppm already in a charged state in a high pressure gas vessel or bomb. Thus, a standard gas of an oxygen concentration below this level is difficult to obtain.
Therefore, in a process or system of heating and producing sensor elements using nickel, or stainless steel, etc., as a resistor or a lead wire, oxygen concentration of the atmosphere of the process is hard to control properly.